A yarn-feeding apparatus of the type to which the invention relates is disclosed in the applicant's own non-prepublished European patent application No. 83 105 136.2 and in the applicant's own international patent application having the application number PCT/EP8300 131. These prior applications describe a yarn-feeding apparatus having a plurality of yarn-feeding wheels which are freely rotatable. These wheels are driven by respective belts engaging a portion of the circumference thereof. These belts are driven in synchronism with the knitting machine in order to supply a plurality of yarns from spools to the knitting machine. Yarn-feeding elements having eyelets for guiding the respective yarns are rotatably supported about an axis perpendicular to the axis of the wheels. The yarn-feeding elements comprise input-guiding elements and output-guiding elements fixedly connected to each other and pivotally supported with respect to said axis. If the knitting machine puts tension on a yarn which is to be fed to the knitting machine, the corresponding yarn-feeding element pivots against the force of a spring, whereby the input-guiding element of this yarn-feeding element urges the corresponding yarn between the belt and the freely rotatable wheel. Hence, the yarn is positively fed to the knitting machine if the knitting machine puts tension to this yarn. During usual operation of the knitting machine, only one yarn is positively fed by the apparatus. In case a change of colour is required, the knitting machine terminates the knitting operation carried out with one yarn, reduces the tension occurring in this yarn and thereby enables the spring to pivot the yarn-feeding element such that the yarn comes in the non-feeding area of the feeding wheel. Thereinafter, the tension in the yarn to be knitted next rises, so that the corresponding yarn-feeding element pivots in its feeding position, in which the yarn comes in the region of engagement of the belt and the feeding wheel. Each yarn-feeding element comprises a sensor generating a sensor signal representing the feeding-position or non-feeding position of the respective yarn-guiding element and thereby representing the respective feeding condition or non-feeding condition of each yarn.
During normal operation, i.e. if no yarn breakage or yarn over-feed or yarn change fault occurs, only one yarn is fed to the knitting machine, so that only one sensor generates a signal. During the changing of the yarn fed to the knitting machine, a simultaneous generation of two sensor signals in case of a knitting machine having an overlapping feeding of the old yarn and the new yarn or the generation of no signal in case of a knitting machine having a non-overlapping feeding of the old yarn and the new yarn takes place during a relatively short period of time. In case of a yarn breakage or a yarn over-feed, no signal is generated by the sensors. In case of a "two yarn"-condition caused by a yarn change, i.e. a fault of simultaneous feeding of at least two yarns to the knitting machine, two sensor signals are generated. Hence it is possible to detect a yarn breakage and a yarn over-feed or a two yarn condition by checking whether a sensor signal combination representing the feeding of no yarn or the feeding of at least two yarns occurs during a period of time exceeding the period of time of the yarn changing operation. For this purpose the apparatus comprises an error detection unit connected to the sensor means for turning off the knitting machine in response to predetermined sensor signal combinations representing a yarn breakage and/or a yarn over-feed and/or a yarn change fault which occur during a period of time exceeding a predetermined time limit. In other words, only sensor signal combinations occurring during a period of time exceeding the time period of a time element of the error detection unit cause the turning off of the knitting machine. Therefore, the error detection unit turns off the knitting machine with a relatively great time-delay which is undesirable, because the knitting machine should be immediately turned off when a yarn breakage or a yarn over-feed occurs so as to prevent damage caused by such an erroneous yarn condition. The detection of an over-feeding of yarn is disregarded during the time period of the time element of the error detection unit, which can result in a so-called "press-off" in the knitting machine, i.e. that the fabric is lost by the knitting needles, which fault takes a considerable amount of time to repair, especially in case of a so-called rib circular knitting machine. Another problem caused by this prior art yarn-feeding apparatus consists in that the time period of the time element defining the time during which the occurrence of a sensor signal combination representing a yarn error is disregarded only exceeds the time for carrying out the yarn-changing operation in case of the normal speed mode of the knitting machine. In case of the so-called "crawl speed mode" of the knitting machine the colour change is carried out more slowly than during the normal speed mode of the knitting machine, so that the error detection unit tends to turn off the knitting machine in case of a yarn changing operation of the knitting machine carried out during its "crawl speed mode". This drawback of the prior yarn-feeding apparatus could only be overcome by extending the time period of the time element for reducing the possibility of erroneous shut-off operations. However, this measure increases the likelihood of disregarding a yarn breakage and/or a yarn over-feed, so that the risk of turning off the knitting machine too late increases.
The present invention is based on the technical task as how to further develop a yarn-feeding apparatus and a method of controlling it such that the reliability of the operation of the yarn-feeding apparatus increases.